In the cable industry, it is well known that changes in ambient conditions lead to differences in water vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside a core of the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High levels of condensed moisture inside a cable core may have a detrimental effect on the transmission characteristics of an optical fiber cable and a metallic conductor cable.
Furthermore, water may enter the cable because of damage to the cable which compromises its integrity. For example, lightning and rodent attacks or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to enter and, if not controlled, to move longitudinally along the cable into splice closures, for example.
Cables for transmitting communications signals must meet industry standards with respect to waterblocking provisions. For example, one industry standard requires that there be no transmission of water under a pressure head of one meter in one hour through a one meter length of cable.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and along the core. For example, a metallic shield which often times is used to protect a metallic conductor cable against lightning and rodent attacks is provided with a sealed longitudinal seam.
Because lightning strikes may cause holes in a metallic shield, it is not uncommon to include additional provisions for preventing the ingress of water into the core. Waterblocking materials have been used to fill cable cores and to coat portions of cable sheath systems to prevent the movement longitudinally thereof of any water which enters the cable. See U.S. Pat. No. 4,176,240 which issued on Nov. 27, 1979, in the name of R. A. Sabia. The use of a filling material, in the form of a grease, causes housekeeping problems, inhibits line speeds because of the need to fill carefully interstices of the cable core, and presents problems for field personnel during splicing operations, for example.
Also, some filling materials have adversely affected the mutual capacitance of the core. This problem has been overcome by a filling material comprising a mixture of a hydrophobic powder in the form of water repellent treated calcium carbonate and a hydrophilic powder in the form of at least one high molecular weight resin rapidly hydratable to form a viscous solution. See U.S. Pat. No. 4,002,819.
Waterblocking provisions inside the core may be other than a filling material. See U.S. Pat. No. 4,946,237 which issued on Aug. 7, 1990 in the names of C. J. Arroyo and P. F. Gagen. A longitudinally extending waterblocking member inside a core tube may take several forms. For example, it may comprise a laminate comprising a powder captured between two tapes. Or, it may comprise a substrate tape which is impregnated with a material. When exposed to water, the impregnating material reacts to swell and causes the tape to prevent the passage of water through the sheath system toward the core and its migration in a direction longitudinally along the cable. In one embodiment, the impregnating material comprises a film of a water swelling or so-called superabsorbent material. In another embodiment, a tape may be treated with a paste comprising a superabsorbent material. The impregnating material may be a polyacrylic acid having a saponification in a relatively wide range or it may be a polyacrylamide. Also, the impregnating material may comprise blends or salts of polyacrylic acid or polyacrylamide, or copolymers or derivatives of the acrylic acid and the acrylamide. Further, the waterblocking provisions within the core may comprise one or more yarns which have been impregnated with a superabsorbent material or which comprise superabsorbent fibers. Also, the waterblocking provisions in the core may comprise a waterblocking tape, which may engage an inner surface of the core tube, and a waterblocking yarn or yarns.
Lately, optical fiber cables have made great inroads into the communications cable market. Although the presence of water within an optical cable may not be detrimental to the performance of optical fibers of the cable, passage of the water within the cable should be prevented as its presence at connection points or terminals may cause problems. For example, the formation of and retention of ice around the optical fibers provide a microbending crushing effect which is known to increase undesirably the attenuation. Thus, protective sheaths must also be water impermeable to prevent or minimize ingress of water. However, even when efforts are made to prevent water ingress, sheath damage may provide pathways for water into cable and upon freezing, attenuation problems will still result. Thoughts have been given, therefore, to the provision of means which will prevent the crushing action of ice upon optical fiber, but to date solutions to the problem have not met with wide acceptance.
Another problem relates to the use of riser cables which connect interoffice and intercity trunks. Such cables include portions which provide vertical riser distribution between two or more floors of a building. Typically indoor optical fiber cables are all air core while outdoor cables include filling materials in the core to provide resistance to water penetration. Desirably, the use of one cable to extend from an outside manhole into a building and upon a riser shaft would result in substantial cost savings because of the elimination of the need for additional splice locations. In order to use one length of cable to extend from a splice location in a manhole outside a building into the building and then to distribution points, the cable must include suitable waterblocking and freeze prevention provisions to satisfy outside requirements which do not compromise the fire retardance properties of the cable needed for internal building use.
Cable manufacturers have resorted to the use of superabsorbent materials which provide the desired resistance to water penetration while not degrading the flame resistance of the cable. As mentioned hereinbefore, in some cables, superabsorbent tapes have been disposed inside the cable core. It has been found that the use of a superabsorbent tape in an optical fiber cable core may result in an increased microbending loss in the fibers because of freezing of the activated superabsorbent gel.
What is needed and seemingly what is not available is an optical fiber cable which includes protection against the flow of water along the cable and which includes provisions for protecting the optical fibers against a freezing environment. Also desirable is a cable which has sufficient flame retardance so that the cable may be used inside buildings such as in risers, for example.